How often it happens—you can walk past something for years and never think twice about it or even notice it at all. Then, once it is called to your attention, you begin to see that something rather often.
That happened to me (again) quite recently. I read an article about the teeth that are found around the edges of certain kinds of leaves. For instance, alder leaves always have noticeable teeth, and even cottonwoods have small teeth that are evident upon close inspection. Of course, for years, I had known about teeth on the margins of alder leaves, but had I thought about them??? Hmmmmm—-no. So now is the time.
Researchers have known for a century that toothy leaves seem to be more common on modern species of trees and shrubs that live in cooler climates, and that association has long been used to reconstruct ancient climates from collections of fossilized leaves. For example, fossil leaves in south-central Alaska were used to support other information showing that Alaska had a much warmer climate millions of years ago. Recent studies, however, suggest that the correlation between toothiness and cool climate is not quite as simple as was commonly supposed, so the modern correlations of the distribution of leaf teeth with climate may not be as precise an indication of past climates as previously thought.
There remains a significant correlation of the occurrence of toothy leaves with cooler climates, however. So then it becomes interesting to consider what leaf teeth do for the plants that bear them.
It turns out that leaf teeth can enhance the rates of carbon uptake early in the growing season. This is particularly important for plants that grow in regions where the growing season is short (i.e., cooler climates). In general, the margins of leaves have high physiological activity early in the season, and toothed margins accomplish more photosynthesis and water transport than untoothed margins. Furthermore, species in the northern U.S. tend to do these things better than southern ones.
Thus, in cool, short-season climates, where rapid early-season growth is especially useful to the plant, leaf teeth are advantageous. A possible disadvantage is increased water loss (all that physiological activity uses water). And so it is presumably no accident that the correlation of leaf teeth with cooler annual temperatures is found where water is not in limited supply; in dry regions, any gain in photosynthesis and energy for growth is outweighed by the problem of water loss.
That’s all well and good, but as usual, it leaves us (me, anyhow) with more questions. For instance, consider our many species of Alaskan willow: some species have toothy leaves and others don’t. (Blueberry species also differ in this way.) Do those with leaf teeth tend to send out leaves earlier in the season, because they can function better than the rest when temperatures are cooler? And even within a single species of willow, some individuals are toothier than others. Do these individuals live in cooler spots or do they only make leaf teeth in cooler years? Then there is the observation that even on a single plant, there is variation in the development of leaf teeth. Are the toothy leaves the first to appear each season, when the weather is cool? And does it matter how big the leaf teeth are? Cottonwood leaf teeth are much smaller than those on alder leaves…
This story is still unfolding, so there is, at present, no last word on the subject. That’s one of the interesting things about science: every idea is continually subject to possible revision as more information comes in.